Mihaela Stefan-Kharicha

Simultaneous Observation of Melt Flow and Motion of Equiaxed Crystals During Solidification Using a Dual Phase Particle Image Velocimetry Technique. Part II: Relative Velocities

A two-camera Particle Image Velocimetry (PIV) technique is applied to study the flow pattern and the equiaxed crystal motion during an equiaxed/columnar solidification process of Ammonium Chloride in a die cast cell. This technique is able to measure simultaneously the liquid and the equiaxed grain velocity pattern as already shown in Part I of this paper. The interaction between the equiaxed grains and the melt flow was explored by means of relative velocities. In single isolated configurations, the settling velocity of equiaxed crystal was found to be 41 times smaller than spheres of equivalent size. The coupling between the fluid flow and the equiaxed crystals was found to be important in areas of high crystal density. Chaotic and turbulent behaviors are found to be damped in regions of high equiaxed crystal density.

Observation of flow regimes and transitions during a columnar solidification experiment

Experimental data for the validation of numerical models coupling solidification and hydrodynamics are very rare. Many experiments made in the field of solidifications are performed with pure metals or alloys (Al-Cu, Pb-Sn, etc) which are opaque and do not allow direct observation of the hydrodynamic. Only the results related to solidification such as grain size and orientation, or macro-segregation are usually used for the validation. The present paper is dedicated to the description of well-controlled experiments where both solidification and fluid dynamic can be simultaneously observed. The important point is the almost purely columnar nature of the solidified mushy region. To our knowledge this is the very first reported macro-scale experiment with almost purely columnar solidification where the flow was measured with a PIV technique. The experiments consist in studying the hydrodynamics during the columnar solidification of a H2O-NH4Cl hypereutectic alloy in a die cast cell. Particle image velocimetry was employed to measure the flow velocity in the liquid bulk. Different flow regimes generated by complex thermo-solutal double diffusive convection were observed. In the beginning of the solidification the solutal buoyancy generates a turbulent flow, which is progressively replaced by the development of stratification from the top of the cell. Later, the stratification leads to the development of a long lasting meandering flow, which filled almost all the liquid region. The kinetic energy of the flow was calculated and it was found out that it decreased with time. The solidification front was smooth and no freckles appeared in the mushy zone. The evolution of the thickness of the mushy zone was measured. As this experiment showed a good reproducibility it represents an excellent benchmark for validation of the numerical models that target the simultaneous prediction of flow dynamics and solidification.

Simultaneous Observation of Melt Flow and Motion of Equiaxed Crystals During Solidification Using a Dual Phase Particle Image Velocimetry Technique. Part I: Stage Characterization of Melt Flow and Equiaxed Crystal Motion

A dual phase particle image velocimetry technique is applied to study the flow pattern during a combined equiaxed-columnar solidification process. This technique is able to measure simultaneously the liquid and the equiaxed grain velocity pattern within an academic Ammonium Chloride water ingot. After the formation of a steady convection pattern, solutal buoyancy together with falling crystals destabilize and break the steady convection flow into multiple chaotic cells. In the beginning of the solidification process, the flow transitioned from 2D to a 3D turbulent regime. The kinetic energy for the flow was calculated during the solidification process.

A scale adaptive dendritic envelope model of solidification at mesoscopic scales

We present the Scale Adaptive Dendritic Envelope (SADE) model of solidification at mesoscopic scales. The new approach is based on the rescaling of the microscopic laws on the desired resolution scale of simulation. The diffusivity, the Gibbs Thomson coefficient, and the fraction which solidifies with the solid front are modified to create large fictitious dendrites (envelope) whose tips growth at the same speed as the microscopic tips. The model is inspired from the methodology of the turbulence models that filter the scales that are smaller than the simulation grid size, such as the Large Eddy Simulation (LES) approach. Here, the solidified structure scales which are smaller than the grid size, such as the tip radius and the smallest arms, are modelled with sub-grid scale models. The envelope growth is coupled with a subgridmodel to account for the contribution of the unresolved secondary arms to the phase transformation. The model is applied to constitutionally undercooled domains with different grid sizes that are larger than the initial secondary arm spacing and much larger than the microscopic tip radius. A similar primary arm spacing is predicted regardless of the grid resolution. However the result obtained with smaller meshes resolve more dendrites branches than with coarser meshes.

Exploration of the double-diffusive convection during dendritic solidification with a combined volume-averaging and cellular-automaton model

A two-dimensional model is built and used to study thermo-solutal (double diffusive) convection generated during the solidification of a binary mixture in a rectangular enclosure cooled from bottom and side walls. In order to catch the smallest solute plumes scale the solidification model simulates directly the envelope of the columnar dendrites with a cellular automaton model. The mushy interior of the dendrite is modelled with a volume averaging method. The model predicts the occurrence of several flow regimes during solidification, such as turbulent, stratified and meandering flows. As a result or origin of the two dimensional meandering flow pattern, the concentration field is found to be organised in horizontal layers of uniform concentration. Those layers are separated by very thin boundary layers so that the concentration varies vertically in staircases.

Simultaneous observation of melt flow and motion of equiaxed crystals during solidification using a dual phase Particle Image Velocimetry technique

A dual phase Particle Image Velocimetry (PIV) technique is applied to study the flow and coupled equiaxed-columnar solidification in an ingot of NH4Cl-H2O solution. This technique is able to monitor simultaneously both velocities of the liquid flow and motion of equiaxed crystals. Before solidification started, a stable thermal buoyancy driven flow, laminar and 2D, was initialized in the ingot. As soon as solidification began, solutal buoyancy together with falling crystals destabilized the initial laminar flow into multiple chaotic cells, and a 3D turbulent regime was achieved. The kinetic energy for the flow was calculated. The interaction between the equiaxed grains and the melt flow was analyzed according to relative velocities. The settling velocity of an isolated equiaxed crystal was found to be 41 times smaller than the calculated one of a spherical crystal of equivalent size. The coupling between the fluid flow and the equiaxed crystal was found to be important in area of high crystal density. Chaotic and turbulent behaviors were found to be damped in regions of high equiaxed crystal density.

Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

It is well known that the growth and motion of equiaxed crystals govern important microstructural features, especially in larger castings such as heavy ingots. To determine the origin of the equiaxed crystals, heterogeneous nucleation, and/or fragmentation of dendrite arms from columnar regions are often discussed. In the present study, we demonstrate that under certain conditions relatively large areas of mushy regions slide downward and form spectacular crystal avalanches. These avalanches crumble into thousands of dendritic fragments, whereby the larger fragments immediately sediment and the smaller proceed to behave as equiaxed crystals. Traces of such crystal avalanches can be seen by conspicuous equiaxed layers in the lower part of the casting. From the arguments in the discussion, it is believed that such a phenomenon may occur in alloys which reveal an upward solutal buoyancy in the interdendritic mush. This would include certain steels and other alloys such as Cu-Al, Pb-Sn, or Ni-Al-alloys. Moreover, the occurrence of crystal avalanches contribute to the formation of V-segregations.